1. Field of the Invention
The present invention relates to non-impact printing apparatus for recording on a moving photoreceptor and a print-head for use therewith.
2. Brief Description of the Prior Art
In the prior art as exemplified by U.S. Pat. No. 4,571,602 printing apparatus is described which comprises a multiplicity of individually addressable and energizable point-like radiation sources, such as LED's arranged in rows for exposing points upon a photoreceptor during movement thereof relative to and in a direction normal to the rows. Driver circuits are provided for simultaneously energizing the radiation sources responsive to respective data bit input signals applied to the driver circuits during an infomation line period. The print or recording head includes a support upon which are mounted chips placed end to end and upon each of which are located a grough of LED's. The driver circuits are incorporated in chips and located to each side of the linear array of LED chips. The driver circuits in this apparatus include a shift register for serially reading-in data-bit signals and for driving respective LED's in accordance with the data signals.
Associated with each driver chip is an input pad onto which a current-level control signal is applied. The signal determines the mean current that is produced by a driver into its corresponding LED's. Reference is made to mean current because, due to fabrication tolerances of drivers and LED's, the actual current through an LED may be up to e.g. 10% larger or smaller than the desired nominal value. In order to set the control signal, four resistors in series are provided between a line carrying a five volt DC voltage and the driver chip. By shorting out one or more of the four different resistors, 15 different possible combinations of current control signals can be made. The current control signal fixed for that driver chip is not fed to the LED's but is instead used to generate a larger actual current that passes through the LED's selected to be energized by the data signals.
A problem with the apparatus described in the aforementioned patent is that 15 levels of control provide an undesirable design limitation. A mean current to one group of LED's that is higher than that to an adjacent group of LED's in the same row will provide reproductions with varying patterns of lines. The eye is very sensitive in detecting low frequency pattern lines in reproductions. While differences in light output between adjacent LED's may not be significant to the eye, differences of mean light output between adjacent groups of LED's tend to degrade image quality, particularly in reproductions of pictorial information.
A further disadvantage of the apparatus of the prior art is that of lacking the ability to provide for easy and preferably automatic maintenance of LED light output to correct for changes in light output resulting from aging or other changes to the print head.
A further disadvantage over the apparatus of the prior art is the offsetting of adjacent LED's into staggered rows and the requirement that circuitry be provided for illuminating the rows at staggered times.
It is, therefore, an object of the invention to provide an improved printer apparatus and print head for use therewith which overcomes the problems of the prior art.